1. Field of the Invention
The present invention is directed to innovations in the manufacture of rock bits. More particularly, the present invention is directed to cast steel rock bit cutter cones into which hard cement cutting inserts are incorporated during the casting process.
2. Brief Description of the Prior Art
Rock bit cutter cones having cemented carbide-type cutter inserts are, generally speaking, used for drilling in subterranean formations under conditions where other drilling cones, such as "milled tooth" cones, would provide relatively low rates of penetration and shorter bit runs. The hard cutter inserts incorporated into rock bits typically comprise cermets, such as tungsten carbide (or other hard metal carbide) in a metal binder phase. The most frequently used cutter inserts for rock bits comprise tungsten carbide in a cobalt binder (WC-Co).
In accordance with typical prior art practice for the preparation of cutter cones having cermet inserts, the steel cutter cones are made first by forging. Thereafter, holes are drilled into the steel cutter cone for accepting the cermet cutter inserts. The cutter inserts usually have a cylindrical base and are usually mounted into the holes with an interference fit. This method of mounting the cutter inserts to the cone is not entirely satisfactory, however, because it is labor intensive. Moreover, the inserts are often dislodged and lost from the cone due to excessive forces, repetitive loads, and shocks which unavoidably occur during subterranean drilling.
With regard to the foregoing, it should be recognized by those skilled in the art that retention of the inserts in the cone is highly dependent on the yield strength of the cone materials. However, in conventional cones, it is not possible nor practical to increase the retention beyond a certain upper limit because increasing yield strength usually results in lowered fracture toughness, potentially leading to cone cracking in service. Therefore, the acceptable upper limit of the yield strength of the cone is limited by the fracture toughness of such material and therefore rock bit insert retention through interference techniques is consequently limited.
In light of the foregoing and in an effort to improve the attachment of the cutter inserts to the cutter cones, the prior art has devised several techniques. For example, U.S. Pat. No. 4,389,074 describes brazing tungsten carbide cobalt inserts into a mining tool with a brazing alloy.
U.S. Pat. No. 3,294,186 describes mounting of tungsten carbide cobalt inserts into rock bits using a layer of a brazing alloy, a nickel shim, and yet another layer of a brazing alloy. This referenced patent is directed simply to a rock bit which has slots into which carbide cutting tips are mounted. The carbide tips are centered in the slots of the prefabricated steel body of the rock bit between two nickel shims, or between two copper shims. A layer of a brazing alloy is placed between the shims and the carbide tip, and also between the shims and the prefabricated steel body of the rock bit. (See Column 1, lines 38-42, of the Buell '186 patent.) The procedure described in this patent, however, is very labor intensive because the brazing is performed in connection with each insert after the cutter cone, having the appropriate apertures for the inserts, has already been formed by conventional techniques.
In sharp contrast o the structure described in the Buell '186 prior art patent, in the present invention the carbide (cermet) inserts are first coated with a suitable metal (preferably nickel or nickel alloy). Thereafter, a steel body of the rock bit is cast to partially embed the inserts. In some preferred embodiments, the cermet inserts are coated first with a copper and thereafter with a nickel layer. Only after these two coats are complete is the steel body of the rock bit cast on the insert. Thus, the steel is metallurgically bonded to the external coating (nickel) during the casting process, where very minor alloying of the steel and the external coating occurs. No intermediate layer of brazing alloy between the nickel and steel is present.
When casting steel, the nickel layer that's adjacent to the steel will also get heated and partially melts. Depending on the rate of diffusion of alloying elements across the nickel-steel interface, an alloy composition with a lower melting point than either the steel or the nickel is formed. At the casting temperature, this phase is molten and solidifies as a new solid phase (a metallurgical phase). This implies that the nickel is metallurgically bonded to the steel. The bonding occurs in the layers that are in intermediate contact. In essence, when two metals (steel and nickel) come in contact at temperature, diffusion of alloying elements occurs from nickel into steel and from steel into nickel. When this happens the layers that are in immediate contact form a metallurgical phase which has a lower liquidous state than either nickel or steel and therefore at the casting temperature, these melt and solidify and form a new phase. Thus, you have a metallurgical bond across that interface. It is not just a mechanical bond between steel and nickel as is common in the prior art. To amplify this further, if you look at a chemical analysis profile across the steel nickel interface you have, on one side, a 100% steel. As you approach the interface you are going to have an alloy which is steel-nickel, rich in steel, poor in nickel. As you go across the interface you will have the same alloy richer in nickel, poorer in steel and away finally to the region that is adjacent to an insert, it is going to be a 100% nickel. So what you really have is a chemical gradient which is also a metallurgical gradient and therefore, it is again, a metallurgical bond. The extent of melting is going to depend on many factors. It is going to depend on the material solubility of the steel and the nickel, and the temperature that the casting is poured, it is going to also depend on the surface oxides present on the steel (contaminants tend to lower the liquidous, but also, affect the mutual solubility of one element in the other).
The prior art has used brazing alloys as intermediate layers. To emphasize these brazing alloys (such as solder) are low temperature materials, which means they do not alloy with the steel or they do not alloy with the substrate since, at these temperatures, melting of steel does not occur. These low temperature alloys are physically, just in surface contact. These brazing alloys form a liquid phase within themselves without mingling with the steel. There is no co-mingling between the steel and the solder (in this case of the braze) so the interface really is not a metallurgical bond. It is a mechanical bond. Again, if you were to use the same analog as was done earlier in which a chemical profile was taken across an interface with the braze, what you will have is 100% steel and then you have a discrete interface, then 100% braze. There is not going to be an intermediate layer where there is a mixture of braze and steel. There is no diffusion of species or co-mingling of species across the interface which makes it a mechanical bond not a metallurgical bond.
Another approach taken by the prior art to improve the mounting of cutter inserts to the cutter cones is to provide a widened, reverse taper base for the cutter inserts. Such inserts are mounted into the cutter cones by embedding the insert in a suitable metal powder and thereafter forming the cutter cone through powder metallurgy processes.
A significantly improved rock bit cutter cone, having strongly bonded cutter inserts, is described in U.S. Pat. No. 4,593,776 which is assigned to the same assignee as the present application. The cutter cone of the invention described in the '776 patent has a steel core covered by a hard cladding formed by a suitable powder metallurgy process. Hard cermet cutter inserts are mounted into holes or openings provided in the steel core. The inserts are metallurgically bonded to the core and cladding during the hot isostatic pressing or like process in which the cladding is consolidated.
Still other techniques for affixing tungsten carbide inserts to drill bodies, tools and the like are described in U.S. Pat. Nos. 1,926,770 and 3,970,158.
A problem encountered in the prior art in connection with cermet cutter inserts, and particularly tungsten carbide cobalt (WC-Co) cutter inserts relates to the formation, under certain conditions, of undesirable metallurgical phases, such as a brittle "eta" phase, in the WC-Co cutter inserts. More specifically, when the cermet insert surrounded by steel, such as a WC-Co insert mounted into a steel rock bit cutter cone, is heated to high temperature, the above-noted "eta" phase is formed in the insert, and the toughness and durability of the insert deteriorates significantly.
As is well understood by those skilled in the metallurgical sciences, the "eta" phase is formed in the tungsten carbide cobalt insert by Fick's Law diffusion of carbon from the insert into the surrounding steel cone matrix. Essentially, the relatively high carbon content of the tungsten carbide cobalt insert, and the high affinity of the adjacent steel for carbon, provide the driving force for the above-noted diffusion, and cause the attendant deterioration of the insert.
Except for the above-mentioned U.S. Pat. No. 4,593,776, the prior art was by and large unable to prevent the formation of undesirable "eta" phase in WC-Co cutter inserts under the above-noted conditions. The foregoing provides perhaps the principal reason why, up to the present invention, the majority of rock bit cutter cones which had WC-Co cutter inserts, had the inserts merely interference fitted in insert holes previously formed in the steel cone of the rock bit.
Moreover, even though it has been considered desirable to have a thermal barrier on the insert for minimizing or eliminating thermally generated fracture associated with casting, as well as retarding or eliminating "eta" phase formation, the prior art was limited in this regard to titanium nitride and titanium carbide coated inserts. The titanium nitride and titanium carbide coated inserts, however, are not bonded to the resulting steel matrix by metallurgical bonds. Therefore, often they are held loosely and, under harsh conditions, are likely to rotate, to be lost, or to initiate cracking in the steel matrix.